The present invention relates to the processing of multiple paths in radars, altimeters, and IFF (identification friend or foe) transceivers. The present invention falls more specifically within the framework of reception with an antenna having at least two reception channels.
The problem is to eliminate the echo signals, i.e. the signals obtained by multiple paths, so as to preserve only the signals received on a direct line, known as direct signals. Various techniques are known for this purpose.
The least expensive, but also the least efficient, of these known techniques consists in making the receiver &lt;&lt;blind&gt;&gt; as soon as a signal has been received, and in doing so for a predetermined length of time. However, this technique is not applicable if the various signals received are close to one other.
A second technique consists in exploiting a Doppler coherence when each transmission and reception device permits it. The coherence of the evolution of the phase of the electromagnetic pulse signal in subcarrier terms is measured during the repetition of the recurrences of transmission. The amplitude and phase measurements of the signals present after detection then result in the discriminating of the echoes, i.e. the indirect signals, as a function of the spectral spread. However, while this technique can be used for example in coherent Doppler radar, it cannot be used with IFF transmitters where it lacks a phase reference.
A third technique that is even better than the second one consists in eliminating the indirect signals by the method known as the complex angle method. However this technique, which requires three reception channels to give good results, is expensive.